Handleiding Celestron Omni XLT 120 (pagina 53 van 62) (Engels)

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F -

Focal Length The distance between a lens (or mirror) and the point at which the image of an object at infinity is

brought to focus. The focal length divided by the aperture of the mirror or lens is termed the focal

ratio.

J -

Jovian Planets

Any of the four gas giant planets that are at a greater distance from the sun than the

terrestrial

planets.

K -

Kuiper Belt A region beyond the orbit of Neptune extending to about 1000 AU which is a source of many short

period comets.

L -

Light-Year (LY)

A light-year is the distance light traverses in a vacuum in one year at the speed of 299,792 km/ sec.

With 31,557,600 seconds in a year, the light-year equals a distance of 9.46 X 1 trillion km (5.87 X 1

trillion mi).

M -

Magnitude Magnitude is a measure of the brightness of a celestial body. The brightest stars are assigned

magnitude 1 and those increasingly fainter from 2 down to magnitude 5. The faintest star that can be

seen without a telescope is about magnitude 6. Each magnitude step corresponds to a ratio of 2.5 in

rightness. Thus a star of magnitude 1 is 2.5 times brighter than a star of magnitude 2, and 100 times

brighter than a magnitude 5 star. The brightest star, Sirius, has an apparent magnitude of -1.6, the

full moon is -12.7, and the Sun's brightness, expressed on a magnitude scale, is -26.78. The zero

point of the apparent magnitude scale is arbitrary.

Meridian A reference line in the sky that starts at the North celestial pole and ends at the South celestial pole

and passes through the zenith. If you are facing South, the meridian starts from your Southern

horizon and passes directly overhead to the North celestial pole.

Messier A French astronomer in the late 1700’s who was primarily looking for comets. Comets are hazy

diffuse objects and so Messier cataloged objects that were not comets to help his search. This

catalog became the Messier Catalog, M1 through M110.

N -

Nebula Interstellar cloud of gas and dust. Also refers to any celestial object that has a cloudy appearance.

North Celestial Pole The point in the Northern hemisphere around which all the stars appear to rotate. This is caused by

the fact that the Earth is rotating on an axis that passes through the North and South celestial poles.

The star Polaris lays less than a degree from this point and is therefore refer

red to as the "Pole Star".

Nova Although Latin for "new" it denotes a star that suddenly becomes explosively bright at the end of its

life cycle.

O -

Open Cluster One of the groupings of stars that is concentrated along the plane of the Milky Way. Most have an

asymmetrical appearance and are loosely assembled. They contain from a dozen to many hundred

stars.

P -

Parallax Parallax is the difference in the apparent position of an object against a background when viewed by

an observer from two different locations. These positions and the actual position of the object form a

triangle from which the apex angle (the parallax) and the distance of the object can be determined if

the length of the baseline between the observing positions is known and the angular direction of the

object from each position at the ends of the baseline has been measured. The traditional method in

astronomy of determining the distance to a celestial object is to measure its parallax.

Parfocal Refers to a group of eyepieces that all require the same distance from the focal plane of the

telescope to be in focus. This means when you focus one parfocal eyepiece all the other parfocal

eyepieces, in a particular line of eyepieces, will be in focus.

Parsec The distance at which a star would show parallax of one second of arc. It is equal to 3.26 light-years,

206,265 astronomical units, or 30,800,000,000,000 km. (Apart from the Sun, no star lies within one

parsec of us.)

Point Source An object which cannot be resolved into an image because it to too far away or too small is

considered a point source. A planet is far away but it can be resolved as a disk. Most stars cannot

be resolved as disks, they are too far away.

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